The importance of chiral drugs in the pharmaceutical market increases with each year. Single stereoisomers on the market have proven to be safer, exhibit fewer side effects, and are more potent than what achiral drugs have been previously able to afford. The fact that pharmaceutical companies can now consider the practicality of marketing chiral drugs is partially due to the ability of synthetic chemists to be able to obtain high enantiomeric excess in asymmetric bond construction.
[R-(R*,R)]-2-(4-fluorophenyl)-b,d-dihydroxy-5-(1-methylethyl)-3-phenyl-4-(phenylamino)-carbonyl]-1H-pyrrole-1-heptanoic acid (atorvastatin, LIPITOR™), whose structure is set forth in FIG. 5, belongs to a class of drugs called statins. Statins reduce the level of total cholesterol and LDL by inhibiting HMG-CoA reductase, an enzyme that catalyzes the conversion of HMG-CoA to mevalonate. Atorvastatin is the most potent of the statins. Atorvastatin contains a chiral β,δ-dihydroxyheptanoic acid side chain that requires a significant effort to produce on a large scale. Fluvastatin (LESCOL™) is water soluble and acts through the inhibition of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase.
The aldol addition reaction, or aldol condensation, is a fundamental organic chemistry method for the formation and dissociation of carbon-carbon bonds. The aldol condensation can create two contiguous stereogenic centers and, consequently, four stereoisomers. Some control over the stereoselectivity can be obtained by the use of preformed enolates with metals. However, these reagents are stoichiometric and require extensive protecting group chemistry. See, for example, C. H. Heathcock, Aldrichim. Acta (1990): vol. 23, p 99; C. H. Heathcock, Science (1981): vol. 214, p 395; D. A. Evans, Science (1988): vol. 240, p 420; S. Masamune, et al., Angew. Chem. Int. Ed. Engl. (1985): vol. 24, p 1; D. A. Evans, et al., Top. Stereochem. (1982): vol. 13, p 1; C. H. Heathcocket et al., in Comprehensive Organic Synthesis, B. M. Trost, Ed. (Pergamon, Oxford, 1991), vol. 2, pp. 133-319 (1991); and I. Paterson, Pure & Appl. Chem. (1992): vol. 64, 1821.
Enantioselectivity can be obtained by using either chiral enol derivatives, chiral aldehydes or ketones, or both. However, recent studies of catalytic antibodies opened ways to obtain enantiomerically pure aldols via resolution. Thus, for some reactions, the problem of complex intermediates may be solved by using relatively reactive compounds rather than the more usual inert antigens to immunize animals or select antibodies from libraries such that the process of antibody induction involves an actual chemical reaction in the binding site. See, for example, C. F. Barbas III, et al., Proc. Natl. Acad. Sci. USA (1991): vol. 88, p 7978 (1991); K D. Janda et al., Proc. Natl. Acad. Sci. USA (1994): vol. 191, p 2532. This same reaction then becomes part of the catalytic mechanism when the antibody interacts with a substrate that shares chemical reactivity with the antigen used to induce it.
The mechanisms of aldol condensation by aldolases have been well characterized. C. Y. Lai, et al., Science (1974): vol. 183, p 1204; and A. J. Morris e al., Biochemistry (1994) vol. 33, p 12291. The enzyme 2-deoxyribose-5-phosphate aldolase (DERA) in vivo catalyzes reversible aldol reaction of acetaldehyde and D-glyceraldehyde 3-phosphate to form D-2-deoxyribose-5-phosphate, the sugar moiety of DNA. Consequently this type I aldolase is widespread in nature. It is the only aldolase that accepts two aldehydes as substrates. Recent studies show that, in certain DERA-catalyzed reactions, product of the first aldol condensation can become an acceptor substrate for a second aldol condensation catalyzed by DERA or another aldolase. Thus, DERA and other aldolases can be used in combination for sequential aldol reactions leading to products with multiple chiral centers, starting from simple, non-chiral substrates. Gijsen, H., Wong, C.-H., JACS, vol. 117, 7585-7591. This enzyme can provide a route to a wide range of potentially biologically active compounds, e.g., the synthesis of deoxysugars such as deoxyriboses, 2-deoxyfucose analogs, and 13C-substituted D-2-deoxyribose-5-phosphate. See, for example, U.S. Pat. No. 5,795,749. It also affords a route to a variety of chiral aldehydes as illustrated in FIG. 6.